text:
Has there ever been a tail-dragger with retractable tail-gear?
This comes from a slightly related question about drag from landing gear.
A tail-dragger is the old-school layout of aircraft, particularly WW2-era fighters. It's also known as Conventional Landing Gear.
The first two landing gear is usually retractable, but the tail wheel is not.
At least, I have never seen such a configuration with a retractable tail wheel. Has there ever been one?
Note: question excludes now more common tri-cycles.

The majority of WW2-era aircraft do in fact have retractable tailwheels; certainly the ones designed within a few years of the war breaking out.

@egid I contest that majority is not the right word to use here, but you do have a point that the number of designs with a tailwheel was not insignificant.

I'm pretty sure it's the right word. Of the most-produced aircraft of the war, the only ones I've been able to confirm was only produced with a fixed tailwheel are the Hawker Hurricane, Avro Lancaster and Ilyushin Il-2. The He-111, B-17, P-51, P-47, Fw 190, Ju 88, Yak-3, and a large number of Bf 109 and Spitfires all were built with retractable tailwheels. I'm sure there are more with fixed tailwheels, but I maintain that the majority of WWII combat types had fully-retractable gear. The DC-3/C-47 was designed a bit too pre-war for me to consider it 'WW2 era'.

@JonathanWalters fwiw, OP didn't mention tricycle gear, so if that's how you read my initial comment I think you misunderstood.

@egid I'm excluding tricycle gear (1 nosewheel, 2 back wheels). Will edit OP.

I'm not sure you need to clarify, since your title (and question) both specifically call out 'taildragger'.

Grumman F4F, Douglas SBD Dauntless, Curtiss SB2C Helldiver, Douglas TBD Devatastor - not retractable. Grumman F6F, Brewster F2A Buffalo - retractable.

@Jonathan, an overwhelming majority of fighters produced during WW2 (after 1940) had retractable tailwheel. Practically all Soviet ones (Yak-1,3,7,9; MiG-3; LaGG-3; La-5,7), except for early Yak-1 revisions and a few minor modifications of others; all Fw 190, most (but not all) Bf 109. These alone outnumber all American-built fighters (of which, still, perhaps the majority had retractable tailwheel). Sometimes they went so far as to cover cowling gaps with paper just to gain a few more km/h of speed!

@Zeus Fighters are all well and good. My statement was regarding "WW2-era aircraft". I present the Il-2, the most produced military aircraft of all time, and second in total number produced only to the civilian C172. We can all cite statistics going either direction, particularly if we qualify the statistics to a particular year, role, type vs quantity produced, etc. The point is, many did—and many did not—have the retractable tailwheel.

@TonyEnnis, the TBD, SBD, and F4F were mostly pre-war types that were outdated at the start of the war. The American air force and all the aircraft were pretty outdated at the start of the war, maybe with the exclusion of the B17 and the B24. The F6F, on the other hand, was the first American plane in which an average pilot could beat the Japanese zero. The SB2C was also kind of outdated, maybe not so much as the TBD, SBD, and the F4F. Also, the SB2C was not a fighter, which were usually designed as light and aerodynamic as possible to increase speed and range.

Even some sailplanes with retractable main wheels have had retractable tailwheels.  The more common arrrangement is a tiny little non-steerable tailwheel where only the bottom sticks out of the fuselage, but I recently saw an exotic (expensive) motorglider with a steerable tailwheel (to facilitate unassisted taxiing) that was fully retractable.

Yes, there are. The Boeing B-17 FLying Fortress is one example of an aircraft designed with a retractable tailwheel.

Source: USAAF via Wikimedia, Public Domain (USGOV-PD)
Another example would be the Vought F4U Corsair:

Source: Wikimedia, cc-by-sa-2.0
Doubtless there are also other examples of aircraft similarly equipped with retractable tailwheels.

Concorde! Actually a "bumper".
Obviously it was there to protect against over-rotation on takeoff, but the relevant item on the checklist was four greens, not three.

@ymb1 The term "tail dragger" is imprecise and contested. The Concorde did have a tail wheel, and did—however rarely—drag its tail on the tailwheel. I think this is a very valid answer, illustrating the vast variation within aircraft design!

'The term "tail dragger" is imprecise and contested.' — it is?

@Egid Yes, by some accounts the term should only be used to refer to conventional gear aircraft equipped with a tail skid—at the exclusion of aircraft equipped with a tailwheel—while other uses or definitions include any conventional gear aircraft—as such aircraft literally or figuratively drag their tail.

As others have mentioned, yes.
Later in World War 2, speed and range became more important than weight and mechanical complexity; an extended tail wheel may be mechanically simpler to build, but adds quite a large amount of drag, affecting range and speed.
There are too many examples to list with links/photos of them all, but a short list of examples include:

Mitsubishi Zero.
P-51 Mustang.
P-40 Thunderbolt.
Later marks of the Supermarine Spitfire (around the MkVIII from memory; I spent 6 years working on a MkV restoration).
Some models of Messerschmitt BF.109. The F models were 'cleaned up' aerodynamically with retracting tail wheels, while the subsequent G models had a fixed wheel due to a bigger tyre for better ground handling, which didn't retract as on the F models. Later G and K models again used a retracting wheel to lessen the drag.
Notable for being an oddball in that it's a jet tail dragger and also equipped for aircraft carrier landings would be the Supermarine Attacker. There's an in-flight photo with the wheels up here.

In addition to the military fighters listed above numerous civilian airplanes with conventional landing gear had retractable tailwheel, including the Beech Staggerwing, the Boeing 307, the Curtis C-46 and many others.

A few more examples are the Soviet Tu-2 front line bomber (fully retractable) and the Il-10 attacker (partially retractable, i.e.: it folded into the fuselage but did not have a fairing).
Tu-2, note the tailwheel bay doors:

Il-10, note the folded tailwheel:

The Bf-109E-4's did not have retractable tailwheel, but the Friedrich which came after the E did have retractable tailwheel. The G had a larger tailwheel to improve the on ground handling that was difficult because of the narrow track undercarriage. This made the tailwheel unretractable. The P-51s, many variants of spitfires, Fw-190, many P-47s, the P-36s, (which were not in service with the US but were with the French), most Yakolev fighters as mentioned above, the F4U, the Zero, the Ki-84 and many others.
The Stuka had no retractable landing gear but part of the reason was that the huge landing gear was used to reduce the airspeed in a dive.
In conclusion, most fighters and many aircraft of WWII had retractable tailwheels.

I think that the P-40 had a retractable tailwheel too

the friedrich models, earlier gustav models of the 109 and kurfurst had retractable gear


text:
What keeps Russian airliners from being as fuel efficient as their Western counterparts?
Peter Kampf points out in his answer on IDing Tupolevs that the Russian transport aircraft designs tend to be quite aerodynamically refined, in some ways better than their Western counterparts on that front.  What kept them from matching the West when it came to fuel efficiency then?  Was it purely a matter of the Russian need to build sturdy aircraft to withstand their rough strips in the middle of Siberia?  Or were there other factors, such as engine technology, in play?  And have the Russians "caught up to us" regarding aircraft fuel economy, or are they still trying to get the hang of building fuel-sipping airplanes?

Do you have numbers? How bad are the Russian fuel economies?

I'd have to dig them up -- it's a common dig on Russian aircraft, though.

Communism, of course.

I'm no Soviet scholar, but after a lifetime (it seems) of reading quite a lot about Soviet space and aviation realms, I can vouch for Kampf's and aeroalias's answer in general. The weak point lies in their engines, although not fully for the "obvious" reason of less quality. I will post my own answer too.

The engines.
Russian engines were for decades behind the state of the art. One reason was manufacturing tolerances. I remember that we studied the Progress D-27 three-spool turboprop and found that just the cast casing could be made 250 kg lighter with Western tolerances. The Russian designers kept their tolerances wider to make sure that the minimum wall thickness would not be undercut.
Also, since Russian designers had less powerful computers available to them, they could rely less on CFD, so the turbo machinery was less refined. Improved engines explain maybe two thirds of the increase in fuel efficiency over the last 70 years.
Another reason is system redundancy. Russian engineers made sure the aircraft would still be operable after being maintained by a substandard maintenance crew. Especially the military designs were built with the goal of maintenance-free operation for several weeks. This thinking was also applied on the civilian side, since all Russian aircraft were designed with dual-use in mind. In case of war, all Aeroflot planes would become military transporters, and Aeroflot crews received most of their training on military aircraft. Why else would designs like the Tupolev Tu-134 have a glazed nose?

Tu-134 (picture source)

So one could try and retrofit a Russian airplane with Western engines to achieve a very efficient aircraft? I know engines are not easily exchangable, but with some work, it should be feasible, right?

@ROIMaison: The Tu-204 is available with RB211 engines, and should be comparable. But now people will complain that the avionics suite is Russian ...

The Russian designers kept their tolerances wider to make sure that the minimum wall thickness would not be undercut.<  Can you expand on this a little? Tolerances as to the sheet metal manufacture? Wall thickness of what?

@TomMcW: The tolerances of the cast part. If the core is misaligned, one side will turn out thicker than the other. After several glasses of vodka, the mold maker at the plant for castings might not get it right down to the last couple of millimeters.

Kampf, can you tell us what year you acquired that D-27 and also what year it was actually built?

The D-27 is a propfan, not a turboprop.

@Sean We can split hairs about semantics all day, or invent new terms to set a design apart like marketing does, but we can also use commonly used terms. The Progress D-27 is a three-shaft gas turbine driving a contra-rotating Aerosila / Stupino SW-27 propeller, and the exit flow of the gas turbine provides little additional thrust. So it is a turboprop, plain and simple. Maybe you got confused with reports about plans to use the Kusnezow NK-93 on the An-70?

This is my own take on this interesting question. I've been reading about Soviet space and aviation realms for what seems like a lifetime. Here is the general knowledge I've accumulated:
The lion's share of resources went to the military, which meant they got the best aircraft engines. Factories are expensive, so they tended to use the same military engines on civil aircraft that optimally needed a different engine design.
Also, most "new" (post-WW2) military planes wanted turbojets for supersonic speed, whereas civil planes would benefit much more from turbofans (a pretty new concept at the time). So many jet airliners got turbojets too for several decades IIRC.
For most post-WW2 Soviet history, petrol was very cheap and so the economic incentive like we see today just didn't exist. This cheap petrol also existed in the US for a time, but the crunch came much earlier (1974 arab oil embargo IIRC, which didn't affect USSR. In fact, the raised oil prices actually benefited USSR selling their native petrol at higher prices). This eventually changed in the 80's IIRC (petrol in Baku drying up), by which time a whole contingent of other economic problems were stacking up and civil aviation just wasn't a priority.
Soviet aircraft place more emphasis on being able to takeoff from poor, dirt fields (for military reasons), which adds weight to the landing gear and maybe fuselage frame.
Soviet aircraft place more emphasis on cold-weather durability, which includes not just lower extremes in the envelope, but also just the general operation in temperatures which are on average colder than in US. This adds weight to engines and other parts (I believe hydraulics? plus any moving exterior joints?).
WW2 completely destroyed the Soviet economy. This is sort of going beyond "the reason is poor engines" and giving a reason for the reason, but it is relevant. The exact opposite happened in America, where WW2 was economically a boon that completely reversed the Great Depression.
It seems like there are some more but I can't think of them. Will edit later if necessary.

Every aircraft manufacturer has their philosophy in designing the aircraft, depending on their experiences (regulatory conditions, economy, availability of materials etc.) The Russians are the largest producers of crude oil and have no incentive to produce fuel efficient aircraft.
Though the Russian aircrafts are aerodynamically equal to their western counterparts, their fuel gulping engines mean that they are as a whole, inefficient. This is because most of the fuel savings realized in aircraft are due to improvements in engine fuel consumption than due to aerodynamic improvements - 69% to 27% (Lee, J. J., Lukachko, S. P., Waitz, I. A., and Schafer, A. (2001). "Historical and future trends in
aircraft performance, cost and emissions." Annual Review Energy Environment, 26, 167-200.).
Improving the fuel efficiency of the engines requires not just a lot of research in aerodynamics, but also in materials (as the efficiency directly depends on the turbine temperature) and new manufacturing techniques. The Russians most probably had decided that it is not worth the cost and effort.
However, this slowly changing as they try to export their aircraft to other countries in the world which demand better fuel efficiency. Still, they have a lot of catching up to do with their western counterparts.

I don't buy this. The USA produces only about 7% less oil than Russia but, somehow, the US is forced to produce fuel-efficient engines but Russia doesn't care? Russian aerodynamicists are good but one of the things that stops Russian jet engines being efficient is a lack of aerodynamics? It just doesn't add up.

You say that "Improving the fuel efficiency of the engines require a lot of result in aerodynamics".

I didn't mean that the Russians are bad in aerodynamics; on the contrary. What I meant was, though the Russian aerodynamics is good, they are being let down by their engines as the engines have a more important part in improving fuel efficiency. The reason why US aircrafts are in general more fuel efficient is that it directly affects the airlines' bottom line. This means that inefficient engines (or aircrafts) will fail to find a customer. The Russians have had no such problem.

Typo.. I meant research.

Right. So making better engines requires good aerodynamics. The Russians are good at aerodynamics, as you say. So why aren't they building better engines?

Better engines depend on a number of variables, with aerodynamics only one of them. I think the two main areas where the Russians are behind are in materials and manufacturing. Russians are not known for having close tolerances, which directly affects the efficiency of the engine. Another thing is that Russian engines (atleast some of them) are thermodynamically (rather than mechanically) limited, indicating that the requirement of better turbine materials.

OK -- I've edited to clarify that point. Feel free to roll back if you think I've messed it up.

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text:
What criteria make an airport suitable for a technical stop for a large airliner?
Assume you have a large airliner such as a 747, and you're taking off from a place, such as Ovda, where you can't load enough fuel to make it to your destination without stopping somewhere along the way.  From all the airports along the route to your destination, what criteria are used to determine what a suitable "somewhere" to stop at for fuel is, assuming that runway size and fuel availability have already been filtered for?

Some factors might be:

Price of fuel at the various airports?
Landing fees?
Are any of the airports so busy that you might have to hold waiting for landing clearance? (You don't want to plan a fuel stop at LHR).
Can you expect long taxis in some places? (All other things being equal, a single runway with a ramp next to it is better news than something like Schiphol or O'Hare).
If you're carrying passengers, are some of the airports in countries with significantly more cumbersome immigration rules applying to technical stops than others? Some may want airport transit visas from passengers of certain nationalities even if they don't disembark.
Political considerations such as the ones mentioned by Terry in the comment.
And, of course, the total expected fuel burn for the two legs, which will depend on your refueling points. Your operations department will make concrete calculations for a number of likely stops, and weather has an influence on top of geography. If you have several possible airports that all happen to be right on the shortest path between your origin and destination, the math will tend to favor a refueling stop roughly in the middle -- this will minimize the total cost-of-lifting-enough-fuel-for-the-rest-of-the-leg.

Finally, if you're planning a recurring scheduled flight, will any of the options provide a business opportunity to take on some extra passengers on days where the demand from the actual origin is low?

I'd add political considerations too as a possible concern. For example, I doubt any aircraft having taken off from Israel would be welcome to refuel at a Saudi Arabian airport since the two countries are technically still at war. At least this was the case back in the 1990s.

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Do helicopters use static wicks?
Fixed-wing aircraft (just about all of them!) have static wicks to dissipate static charge buildups before they can become obnoxious or shocking.  Are helicopters fitted with them? This answer suggests they are not.

Since many helicopters do have static wicks, your question might be better phrased as "why do some helicopters not have static wicks"?

I would even go a bit further. Is there even evidence that some don't?

The R22 and B206, which I am qualified on, do not as standard.  I don't know the answer, but my guess is that since the primary function of wick dischargers is to prevent interference with the avionics, these aircraft simply don't need them.

I understood that static wicks are for IMC flight. The 22 isn't IFR, and I don't think the 206 is either?

No, they are not, and cannot be IFR certified but I have flown variants of both that can fly IFR in VMC.  Neither had wicks so not sure that's it.

I checked out the link you posted showing that they are apparently not fitted and the confusion might arise from why do you need to earth the helicopter?  Well, if it was possible to do this with an aircraft, you would have the same problem.  The wicks do not totally discharge the static, they simply stop a field building up around the airframe.  Air is a lousy conductor of electricity.

@Simon i don't want to belabor the point, but what i asked was not about IFR flight, but IMC flight, when moisture can build up a static charge

@rbp Sorry, "no they aren't" meant that the R22 and B206 are not certified.  I was confirming what you said ;)  Both the variants had full IFR kit and would fly in static inducing conditions.  VMC does not mean no moisture ;)

Static wicks are mostly found on aircraft that fly in Instrument Meteorological Conditions because static charges typically build up when flying in precipitation, not in clear air.
So you won't find static wicks on helicopters that are certified only for visual conditions, like the R-22, R-44, Bell 206, piston Enstroms, etc., but you will find static wicks on helis that are certified for IFR.

[Static wicks] are used on aircraft to allow the continuous
  satisfactory operation of onboard navigation and radio communication
  systems during precipitation (p-static) conditions. Precipitation
  static is an electrical charge on an airplane caused by flying through
  rain, snow, ice, or dust particles. When the aircraft charge is great
  enough, it discharges into the surrounding air. http://en.wikipedia.org/wiki/Static_discharger

that is not to say that a VFR-only aircraft can't fly through rain or snow, but its less likely

IFR-rated helicopters are exceedingly rare.


text:
When is 'heavy' used in callsign?
I have heared at ATC communication that some aircraft uses a heavy word in the call sign.
What does heavy means?
When it is used?
Is it used when aircraft is heavier than 200-300 tons?

Very much related: https://aviation.stackexchange.com/questions/35389/is-there-a-definition-of-light-medium-heavy-aircraft

The rules are slightly different in different countries, but common for all is that the "heavy" designation is used for aircraft in the heavy wake turbulence category, that is, aircraft with a maximum takeoff mass of 136 tonnes or more (see Is there a definition of light/medium/heavy aircraft?).
Some countries require aircraft to use the heavy suffix to their callsign in all radio transmissions. Others only require it on initial contact with a new ATS sector, whereas others still will additionally require it to be used at all times in the aerodrome environment, but only on initial call in the en-route environment. The local rules will be listed in the AIP for the country in question.
Also note that in some places, certain aircraft types are considered to be in the heavy category, even though their MTOM is below 136 tonnes. The typical example of this is the Boeing 757, which is put in the heavy category by some countries because it creates significantly more wake turbulence that other aircraft with similar MTOM.

As to why those rules exist, I read somewhere that it's so ATC doesn't forget to apply the extra separation required since heavies (and supers) are relatively rare.

The term heavy is used during radio transmissions between air traffic control and any aircraft which has been assigned a maximum takeoff weight (MTOW) rating of 136 tonnes (300,000 lb) or more.

More at Wikipedia (source of the cited text) and SKYbrary.

A special case is the 757, which is considered as "heavy" even though its MTOW is below the mentioned MTOW due to the wake-turbulence it produces

Sorry, but this is incorrect. A 757 is not considered a heavy. It was treated differently for wake turbulence, and still is in a less severe way, but a 757 is not a heavy per the MTOW regulation.

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text:
Does ATC clear IFR helicopters for a visual approach if landing at a helipad on a high-rise?
I'm guessing that most helipads on downtown high rises don't have any instrument approaches. If a helicopter is on an IFR to a helipad without any instrument approaches, does ATC just clear them for the visual approach, and do helicopters have their own CTAF frequency for announcing their intentions at off-airport helipads?

There are no "off-airport helipads" because a helipad is an airport. Actually, anywhere an aircraft lands is an airport as far as the FAA is concerned. It would be just like any other IFR to any other airport with no approach procedure: ATC can clear them for the visual approach if they have reasonable assurance that the weather at the airport is VFR, or the pilot can cancel IFR if they determine the weather is VFR. Public helipads have CTAFs just like any other public airports do. They can be found on helicopter charts, or in the Chart Supplement. In the attached chart snippet, you can see that the CTAFs at East 34th Street and Wall Street are 123.075 and 123.05.

One Police Plaza does not have a published CTAF because it is a private airport. Other helipads in the area are also private and not explicitly depicted.


text:
Do pilots know which heading to fly in case of a go around?
If the choice to go around is always an option for the pilot as described in Do pilots need a clearance to go around?, is the heading to go around always well-known at every airport?

This can be overridden by controllers, here's a case where a Ukranian flight was slow to takeoff.  The British Airways flight on short final had to go around.  Ukraine was ordered to stop to protect BA's go-around path.  But they disregarded and took off anyway, putting them in conflict.  Quick action was called for.

Yes. Every instrument approach procedure has a specified missed approach segment. The missed approach will be described in terms of route to fly (heading), altitude, speed and crossing restrictions, terrain clearance and any other required information.
Before approaching an airport, the pilots will conduct a briefing that includes going through the missed approach procedure, so that they are 100% sure what to do if they need to perform a missed approach.
From an air traffic control point of view, we will make sure that there is no other traffic in the missed approach area when an aircraft is conducting an approach. That way there will be no loss of separation if an aircraft goes around.

This holds true for IFR traffic cleared for an instrument approach, but I don't see that it addresses VFR traffic or IFR traffic operating on visual approach not having a published missed.

@JonathanWalters Based on the simplicity of the question I assumed the OP would benefit most from a simple answer.

I'm not downvoting, I'm just pointing out that your simple answer only applies to some of the common cases.

I agree that the distinction between VFR and IFR is important, and a simple statement like "if operating under visual rules, the appropriate procedure is to maneuver visually to enter the traffic pattern unless given other instructions by the controller."  This doesn't really make it more complicated but does show that there is more involved than just one procedure...

All medium sized airports have charts published. Those charts contain every procedure the airport uses. The procedures for departure and for landing.
A go around is also called a Missed Approach. After a go around, the pilot usually flies the published missed approach procedure (MAP) for the approach he is flying. The procedure itself can be seen on the approach chart used. It consists of few, clear statements on how to fly the MAP.
Here an example from my home-airport LOWG. The following is the whole approach chart for the ILS 35 approach:

The red arrow shows the position of the missed approach procedure explanation. Following it is zoomed in:

When the pilot goes missed, he has to do exactly what the procedure sais. Let's go through it.
The MAP explained

CLIMB STRAIGHT AHEAD First of all the pilot should stay on runway track and climb.
WHEN PASSING D-3,0 OEG TURN RIGHT TO NDB GBG He stays on runway track until the aircraft is 3 nautical miles away from the localizer of the ILS, with the designator OEG. When 3 nautical miles are reached, he turns towards the GBG NDB. A NDB (Non directional beacon) is a navigational aid. The pilot can see at which angle he is flying towards the NDB but not how far away he is from it.
CLIMB TO 5000 FT AMSL While flying the pilot should climb on an altitude of 5000 ft Above Main Sea Level.
AND HOLD When he has reached the GBG NDG, the aircraft has to fly a holding pattern. This can also be seen on the chart:

I think the picture is self-explaining. The point to fly the holding, in this case the GBG NDB, is called the missed approach fix.
In this holding the pilot can re-configure the plane for another approach. ATC will guide the plane to the next approach when ready.

Chart Source: Austrocontrol


text:
Can you continue an ILS approach when weather falls below minimum?
If, after passing the final approach fix, weather deteriorates so that the reported values are now below required minima, is it allowed to continue the approach?

You didn't list a jurisdiction so Ill answer for the FAA: Yes you can continue;
Under part 91 you can both start and fly an approach if the conditions are below minimum.
If you are flying under part 121 then you can't start the approach but § 121.651 Takeoff and landing weather minimums: IFR: All certificate holders. is going to be the regulation to reference and allows you to continue if you are passed the FAF.

(c) A pilot who has begun the final approach segment of an instrument
  approach procedure in accordance with paragraph (b) of this section,
  and after that receives a later weather report indicating
  below-minimum conditions, may continue the approach to DA/DH or MDA.
  Upon reaching DA/DH or at MDA, and at any time before the missed
  approach point, the pilot may continue the approach below DA/DH or MDA
  if either the requirements of § 91.176 of this chapter, or the
  following requirements are met:


text:
"Turn right, report heading"?
I can't say anything about authenticity, but here is a quote

Tower: “Delta Zulu Romeo, turn right now and report your heading.”
Pilot: “Wilco. 341, 342, 343, 344, 345…”

Is such an instruction even allowed? And if so, what should the pilot's response be?

Similar, but real life: was once told to "continue present heading" while in a 25-deg bank right turn ...

so why did i get downvoted

again, can the downvoter explain the issue?

The title of the website (Aviation Humor) should give you a hint. It is a joke!
However, there are a few similar situations, one of which is a pilot requesting a turn to avoid weather. That could go something like this:

Pilot: ABC123 request right turn to avoid

ATC: ABC123 right turn approved, report new heading

Pilot: Approved, WILCO, ABC123

Pilot: ABC123 now heading 120

Though normally, the pilot will give a rough estimate of how much they need to turn ("Request right turn 20 degrees to avoid").
Another example is ATC giving a pilot a relative heading change, and asking the pilot to report the actual heading.

ATC: ABC123 turn right by 10 degrees, report new heading

Pilot: Right by 10 degrees, heading 250, ABC123

The reason ATC doesn't just give the target heading to the pilot directly is that in many cases ATC does not know the heading of a flight - only the track. In cases with strong wind especially, the heading and track may differ significantly.
ATC can also tell a pilot to turn without giving a target heading, during non-gyro operations (compass gyro inoperative, so pilot does not know his heading). But in that case, the pilot will not respond (so you won't be hearing "341, 342, 343..."). ATC will then time the turn, knowing that a standard turn is 3 degrees per second.

ATC: ABC123 make all turns rate 1, start and stop all turns on the word NOW.

ATC: ABC123 turn left NOW.

(wait for a specific number of seconds)

ATC: ABC123 stop turn NOW.

Well, I was thinking they wouldn't just make "quotes" up.......

@Abdullah Even if it is a real quote from some prankster, doesn't mean it's approved phraseology. In any case, don't trust everything you read on the internet!

Exactly my point in asking

I've also gotten "turn right 10 degrees and say new heading," sometimes followed by another aircraft also being assigned the heading that I just reported. Thus, they & I are now on parallel tracks, 10 degrees off of what had been my direct course to (whatever waypoint I was navigating to).

@RalphJ True, that's probably the most obvious case actually

@RalphJ but "turn right and report heading" beats that by a fair margin XD

Just yesterday I was flying a SID with a course of 135.  I was flying 150 due to strong winds, and ATC told me to turn right to 150.  "Umm... "


text:
Where does one request a VFR transition route through class B?
Other than staying outside class B until being cleared in, is the point where one requests a transition route important? For example, the SFO B coastline transition is shown as starting up at San Rafael in the north. But one could also fly as far south as SAU below 5000 or Lake Merced below 3000 outside the Bravo before having to ask for clearance. So my question is whether the intent is that one should start at the mapped limits of the transition routes or whether one can ask for them anywhere as long as you're outside the Bravo?

Vaguely related question: the FAA presentation on the latest version of the SFO B shows that the intention for the coastline transition is "at or above 3500". Is this documented somewhere other than in this random PDF from a 2-year-old presentation?

Welcome to aviation.SE! Since you have two separate questions, please put your second one into a new question. The [tour] may be helpful if you're new to the site.

There's an AOPA Air Safety Institute video, "Ask ATC: Am I Cleared for Class B?":

In this and other "Ask ATC" videos, they point out that if you're in VFR flight-following, ATC will not clear you for Class B without your requesting it, even if your current flight path will obviously enter that airspace.
If you are getting ATC VFR flight-following, you should make the request in the course of normal communications, but no later than the point where you can easily divert if clearance is denied.
If you are not talking to ATC, you must first get their attention ("November 12345, request"), then on acknowledgement (when they'll probably abbreviate your call sign, which means you can, too), give your aircraft make/model (which tells ATC your capabilities), relative position (which tells them where to look for your radar return), altitude(s) (level/climbing/descending, which they won't get directly from your transponder), and intentions (your request). This, too, you should do well before the point when diverting around the airspace becomes inconvenient.
(If you're squawking 1200, they'll give you a squawk at the same time, probably an altimeter setting, and possibly altitudes and vectors through the Bravo, so be ready with your lapboard or tablet. Don't hesitate to ask for a re-read, or even to ask them to speak more slowly—in another "Ask ATC" video, they say that controllers often accelerate their speaking pace over the course of a shift without realizing it, they know that VFR pilots aren't as practiced in ATC comms, and they'd much rather you ask for them to repeat more slowly than for you to make a guess.)
So long as you're already handed-off to (if flight-following) or have made contact with the ATC with jurisdiction over the Class B, you can theoretically do it as early as you like.
So there's a bit of a trade-off: later makes it harder to divert, but easier for ATC to have awareness of you. Earlier makes it easier to divert, but ATC may not be ready to clear you if you're far enough out that the traffic situation may change significantly before you enter the airspace.
(Since you're probably significantly slower than most of the aircraft in the Class B, that makes it even more likely that the situation will change.)

Nit: Some Bravos will clear you in without a request, mostly to make their job easier by getting you above all the 1200s crammed below and the resulting constant stream of traffic alerts.


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What's the technique for landing without one set of wheels?
A Boeing 747-400 recently made an emergency landing without the wheels under the right wing:

News story
Photo of the undercarriage
Video of the landing from the end of the runway

What's the standard technique for carrying out such a landing?
What might go wrong? Does it require more runway? Is there a significant risk of dragging one wing on the ground?

While the situation reported was certainly serious. it's worth noting that in the case of the 747, you have both body and wing gear holding up each side of the airplane. Thus it wasn't nearly the problem it would have been in an aicraft with only one landing gear on each side of the airplane, and one of those failed. As a matter of information, had the failure been that of the body gear not extending, that would have been no big deal other than the fact that if the c.g. was greater than 26.6% MAC, the airplane would tip on it's tail once stopped. 26.6% MAC is the location of the wing gear.

These two accidents, that by strange coincidence both happened in Rome, are more interesting in this regard, since A320 does not have body gear like B747.

The techniques vary slightly aircraft to aircraft, and also based on what landing configuration you have.  However, the general gist is to hold off the malfunctioning side from the runway for as long as possible, slowing the aircraft in the process, until forced to put it down on the deck.  This minimizes the contact between the wings and/or fuselage with the ground.
The single greatest danger with these types of landings is the risk of fire. The friction between the runway surface and the aircraft is tremendous generating an enormous amount of heat.  This is particularly problematic when you consider that the fuel is stored in the wings.
Fuel may be pumped to the opposite wing to counter-balance weight and move the fuel away to the fuselage with the highest possibility to have contact with the ground. This technique will shift weight away from the missing wheels and consequently reduce the possibility of fire.

Might want to add something about dumping the fuel before landing to minimize the risk of an explosion.

Indeed it appears they dumped fuel from the right wing only, to make it less likely that wing would scrape the ground.

@jollypianoman Fuel is not dumped to minimise the risk of an explosion.  Indeed, the emptier the tank, the higher the risk of explosion.  Fuel is dumped to reduce weight and, as in this case, to make the CoG advantageous to whatever situation you are in.  An explosion is highly unlikely anyway and there is more risk of fire in landing an overweight aircraft.

It should probably be noted that the wing itself, in the specific case of a 747 but for all aircraft with jet engines below the wings, can't really touch the ground. The first thing to touch the ground would be the engine nacelle. What a landing on a nacelle looks like can be seen here: https://www.youtube.com/watch?v=iPo1mGgK5S4 (YouTube, LOT B767 landing)
Since both the engine housings and the engine mounts are very strong for safety reasons, this works somewhat well (at least in this case).

For aircraft with centerline gear such as the 747, it may be possible to keep the wing off the ground until near zero speed.

@Mark the 747 doesn't have centerline gear. 2 struts under the wings, 2 under the fuselage.

@paul, they're centerline enough that you don't need to keep the low wing flying to keep it off the ground.

Correct - but the term "centerline gear" usually refers to the one in the middle of the fuselage, like on DC10, or A340.

Maybe it's just the way i read it but why "move the fuel away to the fuselage with the highest possibility to have contact with the ground"? Why not move 'away'?


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Flight Following into Class C Airport where I'm Landing
I want to fly into a Class C airport and do a full stop taxi back.  I'll be approaching the airport with flight following.  When ATC hands me off to approach control for the airport I'll be landing at, what do I need to tell approach control given that I'm already on flight following?  I'm guessing that I need to tell them I have the ATIS and that I want to do a full stop taxi back.  It would seem that I would not need to report my position.  Do I need to report my altitude?  It seems I would say, "So Cal Approach Cessna 1234R with information A, inbound for fullstop taxi back."
And on my full stop taxi back, I'll land and then exit the runway.  When I exit I'll be directed to ground, and I'll ask for return for takeoff.  At this point I would also like to ask for flight following to my next airport, but usually I would ask clearance delivery, not ground, for flight following at a Class C airport.  Will ground be able to give me flight following?  Also, I will not have told them that I'm flying a Cessna 172/G because this information was not required to obtain flight following for me because I didn't take off from an airport with clearance delivery.  Is that going to matter when asking for flight following from ground?

You should ALWAYS report your altitude on first call to a radar unit. It is used to verify mode C info.

The answer to this depends a bit on what happens in practice as in my experience flight following does not always orchestrate handoffs as well as say an IFR flight would.
In some cases FF may drop you before you get to the class C airspace and you will have to request clearance into the airspace as if you were flying without flight following.
They may hand you off to the approach controller, and you can always ask for a handoff but FF is under no obligation to provide that. Remember flight following is NOT an airspace clearance and you will need to obtain clearance as you normally would (for class C thats positive radio contact that does not include a diversion or hold instruction). They will generally hand you off with time/distance to deal with this but you should have that approach frequency queued up in the radio.
Yes you should always report your altitude and position (when appropriate) and positive receiving of the weather when calling an approach controller for entry to their airspace assume FF has given them no information. You could say:

So Cal Approach Cessna 1234R at three thousand feet, inbound for a full stop landing at KLAX with information X-Ray

You should report position if you are not handed off and/or you feel as though local procedures require it (maybe FF is provided by a non radar entity), you would add something like

5 miles south of KLAX

Once on the ground you can ask ground, or the tower before takeoff for a flight following handoff. In my experience they will orchestrate this if its a quiet day and they dont have a lot else going on, other than that they are likely to tell you to contact approach in the air and request it.
They dont really need to know anything beyond Cessna 172 which gives them the performance information they need.

Reporting position when being handed off??? Assuming that you're in radar contact & being handed off from one radar facility to another, this seems unnecessary. Center & Approach will always point out IFR traffic to each other in preparation for a handoff, so why not with VFR traffic as well. I'd suspect that checking in with altitude & ATIS and NOT tying up the frequency with the position report would be preferred. But, adapt to whatever the local practices are.

@RalphJ I don’t think do, because usually for VFR flight there won’t be an automated ABI and ACT/LAM exchange

@RalphJ Uncontrolled VFR is normally not coordinated

@expeditedescent VFR Flight Following is a different beast from just VFR (or even CVFR) and normally gets the same handoffs and coordination as IFR.

If they say “contact Approach on 123.45”, that’s a handoff, meaning that your radar target’s ownership has been transferred from one controller to another. Since you’re still “in the system”, you normally only have to check in with your altitude.
However, if you hear “radar service terminated, squawk VFR, try Approach on 123.45”, you have been dropped. This is what happens when a handoff fails, typically because one controller or the other is too busy. You squawk VFR as told and then start all over with the next controller.
When to tell Approach you have the ATIS/WX at your destination is tricky. Only the last controller before tower/CTAF actually cares, but who is last? Look up the airport in the A/FD, and it will list a freq for App/Dep. That is the sector your airport is in, meaning when you get to that freq, add “with [letter]” to your check-in. Except sometimes sectors get combined, so don’t be surprised if you get asked “verify you have information [letter] at [airport]” on an unexpected freq. It happens.
When Approach then hands you off to Tower, Tower will assume you’re full stop by default, so this is when you tell them you want a taxi back, same as you would do on an initial call if you came into a D airport without Flight Following. You don’t need to tell them you have the ATIS/WX, though, since they know Approach will have checked that before handing you off.
When you exit the runway, Tower will tell you to contact Ground for the taxi back, and you can ask Ground whether they can get you a new code for your departure or whether you need to call Clearance. It can go either way depending on their workload at that moment, so don’t be surprised by either answer—or if they give you the new clearance while you’re moving. (Many will ask you to advise when ready to copy, but some will just read it to you right away.) Ground will have the data from your old code in front of them, so you can skip your type, but Clearance won’t know anything about you, so you’d have to give them everything.

As you said, a full-stop taxi-back counts as a full stop in the radar controller's eyes—they will get a new "rolling call" when you take off again. If you'll only be doing a touch-and-go both the tower and the approach controllers will want to know that.


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Any ANSPs uses stack controllers?
The stack controller only controls one or two stack and release aircraft accordingly. Does any ANSP (air navigation service provider) provide this service?

Not so common acronyms should be spelled out at least once.

Airborne holdings are usually handled by ACC or approach sectors, typically manned by ACS or APS rated controllers respectively. There is no separate rating for "stack controllers" as you call them.


text:
Do I need explicit clearance to taxi on an inactive runway?
I received taxi instructions from ground at a class D airport in the USA as follows (I was at ramp): Taxi to runway 04, Right on Bravo, left on 10, right on Alpha (no explicit clearance or hold short instructions for 10 were given). 10 was an inactive runway that intersected 04 with the Alpha taxiway turning off prior to the intersection with 04. I held short when I got to the 10 threshold and asked for clearance to taxi on the runway. The controller followed with Taxi left on 10, right on Alpha, but again no explicit clearance to enter the inactive runway at which point I taxied as instructed. My question is did I need to ask for clearance to taxi on the runway given the original instructions above?

Welcome to aviation.SE! It would be much easier to follow your question if you could tell us which airport, and preferably include an airport diagram. For questions about regulations and procedures, please always tell us which country you're some about. The [tour] may be helpful to get more information on how this site works.

Were there X's painted on the runway?

Not if you're a US Senator.

Thank you @Pondlife! I didn't want to post the specific airport for privacy reasons so I tried to make it as clear as I could without a diagram. I did edit for country location though as per your suggestion.

There were no X's painted on the runway. It was not closed; just inactive.

Yes, you need an explicit instruction (not clearance) to taxi on any runway - and you got one!
The controller told you to "taxi on [runway] 10" which is about as explicit as it gets. If he did not want you to taxi onto the runway, he would have explicitly told you to hold short of runway 10. Note that it doesn't really matter if the runway is "active" or not. In the eyes of ATC, a runway is always a runway. Don't expect the controller to say something like "cleared to enter runway xx" - because it is not a clearance, but an instruction. The phraseology used will be along the lines of "enter runway xx", "taxi on runway xx", "cross runway xx" etc. - never "cleared". The only time you will hear "cleared" and "runway" in the same sentence is in a takeoff clearance or landing clearance.
That said, you did exactly the right thing. If in doubt, stop and ask. Especially when it comes to runways, if in doubt, never assume you have been instructed to enter a runway. Runway incursions are very dangerous, and the controller would much rather clarify an instruction instead of risking a collision.

Although it is not a "clearance", when ATC gives an instruction like, "Cross runway 21", the common pilot response is "cleared across runway 21". (It may not be proper phraseology, but I do hear it alot)

FAA-land used to allow crossing an "inactive" runway without an explicit instruction. That changed several years ago, but confusion about the new rules persists, even among instructors.

And when you receive instruction to enter a runway, your readback must include that instruction and your full (non abbreviated) callsign.

J. Hougaard is correct. In “FAA-land” you also need clearance  from ATC to be on a runway regardless of if you are in an airplane or not. You do not have to hear the words “clear” or “cleared”.
Typically, the instruction you get will be to “cross”, “taxi”, “back-taxi”, or “Line Up And Wait”. I’ve even heard the probably non-standard “do a 180”. If ATC does not want you to enter the runway, they typically say “Hold”, “Hold at” or “Hold short”. In my experience, that will be the last instruction in their string of instructions.
They typically will delay giving you further instructions until they are ready to clear you to enter the runway. I believe that is not always the case. ATC may be allowed to give you instructions in their entirety the first time to allow you the opportunity to write them down prior to beginning your taxi. It is still your responsibility to stop your aircraft wherever they told you to Hold until cleared to proceed.
As a paratrooper jumping onto an active airfield, we had spotters in touch with ATC stopping us at the edge of the runway until they received clearance for us to cross on foot. Nothing like staring down a Southwest Airlines 737 on takeoff coming straight at you at over 100 knots (actually happened). Those engines are much lower to the ground than a C130. I was not thinking of runway incursion reports when the wing passed over our heads.

Side note, but why would they even allow parachute jumping over an active airfield?  That just seems like an inherently bad idea from the get-go...

@DarrelHoffman - We did it quite a lot on military airfields, civilian/military combined/mixed use airfields, and smaller commercial airports (only during air shows). It only takes about 5 minutes for an Air Force FAC/TAC/controller or an Army Pathfinder (in the past) to take total control of the airspace for the jump. And, only 2 minutes for the actual exit of over a platoon-sized element of soldiers from a C141. Then, control of  the airfield is given back to regular traffic. Paratroopers will tend to avoid the hard asphalt runway if possible, and land in the abundant amount of turf.

@DarrelHoffman -Slightly before my time, they did it “Just Cause”.

@DarrelHoffman All the civilian jump zones near me are at airports. That way, they don’t have to go find the jumpers and bring them back to the airport. The jump plane beats them to the ground and picks up the next load, then takes off again as soon as the air is clear. It’s about as nuts as you’d expect from people who jump out of planes for fun.


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What does the ATR72-600 MFD System display look like?
I'm curious how MFD System display looks in real life in ATR72-600, especially electrical mode. I've found three different images showing it, but they seems to be ambiguous. Last one attached comes from FCOM, two others from other docs. Which one is closest to real life? Buses symbols should be as rounded corner rectangles or simple one? States (like "OFF", "SHED") should be out or inside rectangles? Maybe it depends on software version or some specal mode? Maybe someone has real life photos showing it? Any info is highly appreciated.

Two "black" images come from "User guide for Engineers" from Thales Avionics, last one as i said from FCOM.

Two actual photos of the electrical systems pages (AC and DC respectively). Hope it helps.

Sorry for being late. Thanks for these priceless photos! Right now everything is much more clear and i could move forward with work ;).

For me, it looks like the first picture is something like a screenshot. It looks real. (A bit blurred and pixelated)
The second one looks like a graphic made in a graphics program. There can be small deviations from the original screen design, because it's only for informational purposes.
The third one is a black/white graphic that is made for printing (in the FCOM). I think it should be showing the real MFD quite close to reality because pilots must see it as in real life. But this saves printer color and has more contrast.
I am pretty sure, the first one is a real screenshot. It shows commonalities with the drawing in the FCOM and it looks real.


text:
What is this tiny slat for?
The one labeled as "1" in the picture. And I'm having doubts about "2", it is an air supplier for cabin, right? Or am I wrong?

Thank you.

The item 1 is a Krüger flap, a leading edge device which allows the wing to operate to a higher angle of attack. The wing-root fairing makes a small triangular segment necessary; outboard of it slats are used.
The item 2 is a NACA inlet for cabin air, as commented by @mins.

In the wiki regarding Krüger flaps only Boeing is mentioned. Does it mean the Airbus is using slabs only?

1) Appears to be where the inboard leading edge slats retract into.
2) Ram air inlet for the air conditioning/equipment cooling.


text:
Can a passenger force a Go Around?
Sorry if this question is too stupid, but I wonder what would happen if a passenger on the first row just behind the cockpit (ie. on a 737) starts screaming "GO AROUND" when the plane is about to touch down. Can it confuse the pilots?

Related: Can pilots hear their passengers clapping on touchdown?

What would happen? The passenger would probably be taken away by police after landing.

I don't know why this questions is receiving downvotes, it's a perfectly reasonably question from someone who's a little new to aviation, there's no need to downvote.

Maybe if the passenger shouted "Hey, this is Ellsworth AFB, not Rapid City Municipal!" :) Seriously, though, how do you not notice that there's a line of supersonic nuclear bombers on the ramp instead of a passenger terminal?

@reirab, they're closely-spaced airports with their single runways pointing in about the same direction, and Ellsworth is more prominent.

If the pilots don't see supersonic bombers lined up on the apron, they're probably not there.

"Can it confuse the pilots?" is not the same question than "Can a passenger force a Go Around?". The answer could also be different for an aircraft with an open cockpit like a Twin Otter.

@reirab because they were landing at night (evening take off, wasn't a long flight) and most AFBs tend to not keep spotlights on the supersonic nuclear bombers parked on the apron. ;) The real question is why ATC at Ellsworth didn't question the random, large plane heading into their airspace.

@FreeMan It landed at 8:42PM local time, which is still light out this time of year in Rapid City. Looking at the flight path, though, it looks like they would have been looking West as they approached the area and Southwest to see the field as they were flying the downwind, so they probably had the sun in their eyes. It's pretty clear they made the mistake before they entered the downwind, since they flew it parallel to Ellsworth's runway instead of to RAP's.

@FreeMan They're both Class D fields, so their airspace is small and Ellsworth's Tower probably noticed it too late to do much about it, since the flight wouldn't have been on their frequency. You'd think RAP's Tower would have alerted them to the fact that they were busting Ellsworth's D, though, if RAP's tower was still open then (I'm not sure what time they close.) They probably should have paid more attention to "BE ALERT DO NOT MISTAKE ELLSWORTH AFB, LOCATED 6.5 NM NNW FOR RAPID CITY RGNL" in the A/FD, though.

@reirab - ah, I just went with CNN's "evening departure" and didn't look up exact times. Also, didn't pay enough attention to notice that this just happened a day or two ago. Most of the comment references are to rather older events.

Neither will there be a go around nor the prison van will change course

Of course a passenger can force a go-around, if he has a valid argument.

Barring the Ellsworth AFB incident, what would the passenger be able to see that the pilot wouldn't have a better view of?  Between cockpit instruments and the front windows I'm not sure there are any situations were the passenger would have a better idea of an issue than the pilot.

Not likely. Pilots wear headphones, so we cannot hear the idle prattle of the pax in the hold. In any case, we don't take orders from passengers. Think about that for a second: would you like it if aircraft pilots followed the instructions of a crazy screaming passenger? What if there were TWO crazy screaming passengers giving contradictory commands? Which one should we obey?
In any case, nowadays cockpit doors are closed during flight anyway.

The headphones thing is the key here I think. While I obviously know you won't take orders from passengers, my point was, if hearing "go around" at the right time loud enough might trigger an unconscious reaction. I am a frequent flyer and know the door is closed, but on a 737 the first row is just one or two meters from the cockpit door, so just wondered if a loud scream could be heard inside :)

Maybe you could bang in morse code on the door?

I would say that their is zero chance that the pilot flying, even unconsciously, react to a voice shouted from the cabin.

@Simon - I'd be more worried about the unconscious pilot flying the plane than any chance of a go around. In fact, under those circumstances, I think the go around would be preferable than the inevitable crash caused by the unconscious pilot.

@Johnny Still, the unconscious pilot isn't going to react to the voices from the cabin. Simon... isn't wrong... http://i.imgur.com/t9Kkt8h.jpg

Many GA aircraft do not have a cockpit door, so you may be more susceptible to a crazy screaming passenger.

Pax are definitely distracting for GA pilots in small aircraft, but we're still not going to take orders from them. However, I could easily see going around because of one. On one of my first flights after receiving my PP ASEL, my right seat passenger decided to answer his phone while we were on short final. It was just distracting enough that I had a hard landing, flared early and dropped it in. I was very surprised by how distracting it was, to the point that if it happened again and I noticed myself paying any attention to the passenger, I would go around.

Unless he’s shouting “cow on the runway”, I don’t even hear my copilot on finals as he mutters “too fast/slow”, “too high/low”, “left a bit”, etc., but I hear fine as we taxi and he says “Coffee’s on me”.

Hah! Cow on the runway! Like that would ever happen!

Welcome to the site! This is definitely relevant, but it doesn't really answer the question. If you aren't familiar with how stack sites work then you may want to check out the tour

Aircraft do hit cows on runways - http://edition.cnn.com/2013/08/08/travel/plane-hits-cow/. In Australia, sometimes strips have to be overflown to clear them of kangaroos.

In India, I've been a passenger on a 14 seater plane, and it was normal practice to over fly the runway to make sure it was clear of cows (and other things) before landing.

No, but the passenger is likely to be arrested for causing such a disruption and face serious charges. The authorities don't react kindly to that kind of crap in a post 9/11 world.

The chosen answer is correct so far, no doubt. But in my eyes, it lacks some generality and an important point:
While it is certainly true for a 737 that a pilot won't hear you and therefore take no action, I'd add that is can be true for smaller aircrafts when you hear the passenger as they may even can speak over the intercom (say a Cessna 172). As a pilot, you have to decide, if you realize that it was a passenger screaming "go around", if you take action. You train a lot of things as a pilot, one of the most important one is to make a go around as fast as possible if anyone screams it (normally co-pilot, flight student etc) and you do not think about whether it is right or not (you assume he had a good reason) because there is no time, you better focus on the maneuver.
The conclusion is nearly the same as mentioned in other answers with the small addition that, if the pilot hears you, you may trigger it's trained reflexes and he will make a go-around.

Wow Mayou that was exactly the point of my question :)

I would think it depends on the pilot. Some will react reflexively, others will consider the source. i.e. if my 9 year-old daughter shouted "go around" while we were on final, I would probably say, "Why?" And then I would wonder where she learned the term "go around" from...
On the other hand, if we're on final and she says, "Hey look, there's a dear on the runway!" then I will definitely take notice and likely go around.
My point is, the passenger would be better off to shout the reason instead of shouting "go around" if they're trying to prompt a change of course.

You're absolutely right! That's why I wrote  the "may", because it depends so much on who says it, how he says it, who's the pilot and on the situation, e.g. how stressed are you. If you're already under high pressure, say high crosswind, rainy, a lot of traffic and someone yells "go around", you will probably not be able to think over it.
And completely agree that no passenger actually should scream "go around" but the reason.

Yup - "force" a go around? No, not without grabbing the controls or having a chat with ATC on the sly. "Cause" a go around? Yes. There's plenty of cases of incidents where passengers have provided information that the pilot needed, including one where a passenger spotted that the wrong engine had been shut down, and the one on fire was still burning and turning.

good point on the training reflexes.

@hemp: She probably learned it from you.

If there is a legitimate need for a go-around like an emergency that the pilots can't see but the passenger can or landing at the wrong airport, then it might influence the pilots. However, that might not happen because of the conditions described by @Tyler Durden's answer.

authors: Егор Антоневич


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What is the differences of ADS-B and geometric height?
Why does ADS-B and geometric height (calculated height) differ from each other?
If the geometric heights are calculated at night where the temperature is lower, then the result should be different during the day, right?
Is ADS-B with night or day calculated height smaller?
What is the more accurate result to be referenced?

I am having a really hard time trying to understand what you mean, could you please consider improving the wording of your question and making it clearer?

ADS-B is a system, not a measurement. This is like asking why a car and speed are different from each other. I suspect you would like to discuss ADS-B reported altitude and ADS-B reported geometric height or otherwise measured geometric height, but please state so.

ADS-B, and the older Transponder Mode C, report pressure altitude. That is static pressure where the plane is expressed as altitude at which that pressure occurs in international standard atmosphere.
In low pressure area, the same pressure density corresponds occurs at lower geometric altitude with similar difference at all altitudes. In lower temperature, the air is denser and the isobars closer together, so the same pressure altitude occurs at lower geometric altitude and the difference increases with altitude.
However for purpose of traffic separation, pressure altitude is always used. Due to physical properties of air, the pressure always decreases with altitude and at the same altitude changes only slowly with position, so we can always rely on close aircraft observing sufficiently different pressure altitude being at sufficiently different geometrical altitude.
The reason is that while barometric altimeter is simple, accurate to less than 100 ft and every aircraft has it since the early days of aviation, there was no way to measure geometric altitude before selective availability was turned off in GPS in 2000.
Geometrical altitude is altitude above the reference geoid (usually WGS84), a.k.a the sea level. It is only used internally in EGPWS where it is provided by GPS.
In contrast height is in aviation always used to mean height above terrain. It may be measured directly using radar altimeter or it can be estimated from pressure altitude by correcting for sea-level pressure at the nearest airfield.
Radar altitude is used in GPWS as a backup datum and in older such systems  from before GPS was available (those systems could warn if you were too low, but not if you were approaching high ground, because the radar altimeter only measures down while the new GPS-based systems have a terrain map and compare the GPS position with it) and is shown to pilot for reference, but it is not broadcast anywhere.
Approaches are normally flown in reference to the barometric altitude that is corrected for pressure at airport (the altimeter has setting for reference sea-level pressure, QNH), but not temperature. So the barometric altimeter shows true altitude at the airport, but when it's cold, it shows more than true altitude above. That's why some approaches have minimum temperature.
However, both Mode C and ADS-B broadcast pressure altitude, i.e. as calculated for standard pressure of 29.92 inHg/1013 hPa.

In aviation altitude usually refers to distance between MSL(mean see level) and aircraft and height to the distance between aircraft and terrain. Barometric altimeters are used to measure altitude. But they have a lot of errors. To get correct altitude you have to set correct pressure and correct for other errors like temperature. Temperature corrections are only used if it's cold and you are in danger of hitting obstacles. So you will never have exact altitude not to speak about height because altimeter knows nothing about terrain. Transponder just reports number based on standard pressure setting. So this number can differ by hundred of meters from real altitude.

ADS-B Airborne Position squitter carries altitude which might be uncorrrected pressure altitude or GNSS HAE. There are different format type codes for each altitude type. You might check Technical Provisions for mode s Extended Squitter document for this.
To correct your implication, ADS-B does not only use pressure altitude.


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SERA.8012 and ATS.TR.220 Wake Turbulence Separation application
I need small clarification on current European documents SERA and Part-ATS.
Based on SERA.8012(a):

(a) Air traffic control units shall apply wake turbulence separation
minima to aircraft in the approach and departure phases of flight
in any of the following circumstances

Part-ATS ATS.TR.220 states different time-based and distance-based separation intervals, but, for instance, not a time nor distance interval is specified for LIGHT acft following LIGHT acft. For example, 2 Cessna 172 flying IFR, ILS approach to the same runway, so Wake Turb Separation to be applied (exception made for VFR-VFR and IFR making visual approach following IFR traffic).
In RECAT-EU (Part-ATS AMC7 ATS.TR.220) distances for non-covered cases are marked as:

(*) means that the separation minimum to be applied is the horizontal
separation minimum based on an ATS surveillance system (established in
accordance with AMC1 ATS.TR.210(c)(2)), and should remain compatible
with runway capacity.

but for time intervals it's still not clear.

In case there is no published distance/time for pair of aircraft (like LIGHT-LIGHT), what separation shall be used in airport not applying RECAT? I would assume the interval always shall be the minimum of zone (e.g. 3 nm for TMA/CTR), and reduced/increased only when Wake turbulence pair exists. But I have no proof in docs, it can be interpreted differently.
SERA describes "approach and departure phases of flight", then Part-ATS covers either departing, or arriving cases. Shall the wake turbulence separation be applied between traffic just taken-off and about to land on the same runway? My gut feeling about safety says yes, wake turbulence separation shall be applied to ALL traffic within 1000 feet/same runway, but I see no exact case in the Rules.
In case of class D airspace, where IFR and VFR are informed about each other, imagining LIGHT Cessna 182 IFR coming behind LIGHT Cessna 152 doing VFR landing, there is no wake turbulence separation case in documents, and there is no separation by airspace class, so they can fly in whatever interval literally? (taking into account runway occupancy time and landing clearance allowance, for sure).

p.s. At some airports such shady cases are covered as "In any case separation is not specified by docs, minimum separation shall be 3 nm for all traffic coming to the same runway", but I do know, that not all the airports have it stated in local rules.

Different separation minima apply in different airspaces and different situations. For radar environments, 5 or 3 NM are common values (sometimes 2,5 NM on final approach). Vertical separation is usually 1000 ft. In non-radar environments, procedural separation minima apply, for example aircraft tracking away from a radio beacon have to be on tracks diverging by a certain number of degrees and a certain distance from the beacon. These minima always apply, and have nothing to do with wake turbulence.
Wake turbulence separation minima are applied in addition to normal separation minima. This means that whenever the wake turbulence separation minima are larger than the general applicable minima, those need to be applied. For example, the general separation minima for an airspace might be 3 NM, but in the case of a medium aircraft following a heavy, this has to be increased to 5 NM. When the wake turbulence separation minima are smaller - or non existing, such as the case with a light following another light - the controller only has to apply the general separation minima (for example 3 NM or 1000 ft). You can never reduce the required separation below the general minima applicable for the airspace.
2.
Yes, wake turbulence separation must also be applied between a preceeding departing aircraft and a succeeding arriving aircraft. The logic here is that the arriving aircraft might execute a missed approach at any point, in which case wake turbulence might become an issue.
3.
Correct. No specific airborne minima apply, but the controller must ensure there is never a risk of collision and that the runway separation minima are applied (could be reduced runway separation minima, if implemented).
P.s.
There are ALWAYS separation minima for ALL parts of controlled airspace. If you can't find them in the local rules, it is because they are not different from the minima described in the general rules (i.e. EU 373/2017 or national regulations). There is nothing "shady" about that.

Logically I do agree that separations should be applied in addition. Either radar, or wake turbulence. But then in class D that means there is no separation IFR-VFR at all, so even wake turbulence separation is not applied (since..it's separation still, right?), and only "caution wake turbulence" is used?

@BohdanBessonov Wake turbulence separation is still applied during landing and takeoff, regardless of the airspace class.

so it's applied always, but in case there is no matching pair (like Light-Light or Medium-Medium) - zone separation is applied (if it's applicable). Thanks


text:
How is the emergency exit row handled when there are no passengers aboard eligible to sit in an exit row seat?
Based on Roddy's question about nobody volunteering for the exit row, I can imagine a related but more extreme scenario: what if there are no passengers on the plane who are eligible for an exit row seat? (Say if you had a charter flight full of seniors.)  Do the exit rows simply remain empty in that case? How would the operator ensure that the overwing exits are usable in case they are needed?

A charter full of kids probably wouldn't be the best example, since children usually aren't allowed to fly unaccompanied. There would always be either teachers or parents on board.

@Nzall -- good point

It could be a flight full of kids and seniors! (I took many a road trip with my grandparents, neither of whom would have been eligible for an exit row seat.)

@Nzall In the US the cutoff for unaccompanied children appears to be at 5 years for direct flights.  Not sure if it's a law or they just all standardized on the same limit but Google shows the same floor for American, United and Southwest airlines.  (No selection bias other than these being the first 3 results I got in Google.)

@DanNeely Even if they're registered as unaccompanied, it would be VERY unrealistic for an entire plane to be filled with unaccompanied children. Almost sounds a plot for a bad kid's movie.

In such an extreme case, the row would be unoccupied.
Nothing says that the row has to have people in it; I've had flights with under a dozen passengers, and if everybody had seats up front, the row would be empty, and that's okay.
You can't have unqualified people in the Emergency Exit row -- that's the limitation. Nothing requires the FA's the move qualified people into the row. That typically does happen (and as has been pointed out elsewhere, that row is often desirable due to legroom), but it's okay if the row remains empty.

Nothing requires the FA's the move qualified people into the row. That typically does happen I've read in several different places that this does NOT usually happen. Due to the extra leg room those seats often cost slightly more and several people have said that they asked if they could move there since it was unoccupied and the answer was an unequivocal "no."


text:
Can an appropriately-fitted UAS (of any size category) be flown in Class B airspace?
If I fitted an Unmanned Aircraft System (UAS) with an operating Mode C transponder and maintained two-way radio contact with ATC, could my UAS be cleared into class B airspace in the USA?
(Inspired by:
What legal actions would be taken against a drone pilot if they flew a drone into class B airspace?
)

Are we talking about a toy RC aircraft here or, say, an MQ-1 Predator? In particular, are you asking about a hobbyist aircraft, a commercial one, or a government-operated one? The rules for each are very different.

Let's start with a say -- 10lb drone here.  (A MQ-1-sized UAS is a bit of a different story, ofc)  As to the operator, I'm mostly interested in commercial or public uses.

When you say "public" uses, do you mean usage by the public (e.g. hobbyists) or "public" in the sense of the government sector (the FAA uses the term to mean government use, but I just wanted to clarify.)

@reirab -- I'm using "public use" in the same sense as the FAA here :)

In general, no, you can't.

Public (Governmental) Use
The FAA maintains a page on Public Operations of UASs. There are a couple of ways governmental organizations (excluding the armed forces) can fly a UAS:

It can fly in accordance with the limitations set out in Public Law 112-95 Title I, Subtitle B, Section 334 (c)(2)(C), which are that the FAA will:

(C) allow a government public safety agency to operate unmanned aircraft weighing 4.4 pounds or less, if operated—

(i) within the line of sight of the operator;
    (ii) less than 400 feet above the ground;
    (iii) during daylight conditions;
    (iv) within Class G airspace; and
    (v) outside of 5 statute miles from any airport,
    heliport, seaplane base, spaceport, or other location
    with aviation activities.

So, obviously, that's a no-go for Bravo.

It can apply for a Certificate of Waiver or Authorization from the FAA, which "permits public agencies and organizations to operate a particular aircraft, for a particular purpose, in a particular area." According to the FAA,

The FAA works with these organizations to develop conditions and limitations for UAS operations to ensure they do not jeopardize the safety of other aviation operations. The objective is to issue a COA with parameters that ensure a level of safety equivalent to manned aircraft. Usually, this entails making sure that the UAS does not operate in a populated area and that the aircraft is observed, either by someone in a manned aircraft or someone on the ground to ensure separation from other aircraft in accordance with right-of-way rules. Common public uses today include law enforcement, firefighting, border patrol, disaster relief, search and rescue, military training, and other government operational missions.

So, unless you have a really convincing argument, it sounds like the FAA is very unlikely to include in your COA authorization for your UAS to enter Class B (or get anywhere near it, for that matter.)

Civil, Non-Hobbyist Operation
The FAA also maintains a page on Civil Operations of UASs. That page summarizes how civil UAS operations can be approved as follows:

There are presently two methods of gaining FAA authorization to fly civil (non-governmental) UAS:

Section 333 Exemption – a grant of exemption in accordance with Section 333 AND a civil Certificate of Waiver or Authorization (COA); this process may be used to perform commercial operations in low-risk, controlled environments. Instructions for filing a petition for exemption are available here.

Surely, operating in Class B isn't "low risk" (though I suppose you could argue it's "controlled,") so this certainly seems like a no-go.

Special Airworthiness Certificate (SAC) – applicants must be able to describe how their system is designed, constructed, and manufactured, including engineering processes, software development and control, configuration management, and quality assurance procedures used, along with how and where they intend to fly.

a. SAC in the experimental category – may be used for civil aircraft to perform research and development, crew training, and market surveys; however, carrying persons or property for compensation or hire is prohibited. FAA Order 8130.34 is used by FAA inspectors to issue experimental airworthiness certificates and special flight permits to UAS. For more information, please contact the Airworthiness Certification Service, AIR-113, at 202-267-1575.
If the FAA determines the project does not present an unreasonable safety risk, the local FAA Manufacturing Inspection District Office will issue a Special Airworthiness Certificate in the Experimental Category with operating limitations applicable to the particular UAS.
b. A UAS type and airworthiness certificate in the Restricted Category (14 CFR § 21.25(a)(2) and § 21.185) for a special purpose or a type certificate for production of the UAS under 14 CFR § 21.25(a)(1) or § 21.17. For more information, please contact the Los Angeles Aircraft Certification Office at 562-627-5200.

(Bold emphasis is mine, italics emphasis is from the FAA.)
For UASs with a Special Airworthiness Certificate, FAA Order 8130.34C specifies in Chapter 3, Section 1.3 (Flight Test and Operating Area) that:

All proposed flight test and operating areas outside of restricted airspace must be approved by the FAA. Flight test and operating areas will be coordinated with the air traffic component of AFS-80.

and that:

The flight test area and operating area must be over open water, or sparsely populated areas, having light air traffic. The FAA is required to evaluate each application to determine that the proposed flight area does not exceed that which is reasonably required to accomplish the program.

Certainly, Class B doesn't qualify as "sparsely populated areas, having light air traffic," so this, too, seems to be a no-go.

All of those provisions seems to be written with a com-less aircraft in mind.  Is there anything that talks about a UAS where the pilot is able to keep in contact with the local tower and has equipment so it can accurately report location, speed and altitude?  Basically everything a real plane is required to have, just with the pilot on the ground instead of in the craft.

@JayCarr As far as I know, these regs were written with all UASs in mind. The definition of "unmanned aircraft" and of UAS used in PL112-95 is just any aircraft that doesn't have provisions for a human to fly it from on-board the aircraft. There was no listed exception for when the pilot is in contact with ATC. I would assume that specific communication procedures for a given operation would be part of the COA or SAC for that UAS.


text:
How is a "conflict" between aircraft predicted and determined?
To answer my question about the prediction and determination of conflicts, we need a clear definition of the term. As far as I know is the term conflict often mistaken with the term loss of separation.
Let me start with the definition of conflict:

Converging of aircraft in space and time which constitutes a predicted violation of a given set of separation minima. (emphasis mine)

Whereas the definition of a loss of separation is:

A defined loss of separation between airborne aircraft occurs whenever specified separation minima in Controlled Airspace are breached. Minimum separation standards for airspace are specified by ATS authorities, based on ICAO standards. (emphasis mine)

To summarize the matter, a conflict between two aircraft occurs, not when there is a loss of separation, but when there is an impending loss of separation.
Seems pretty straightforward, right?
The challenge is that little word predicted. Who predicts the converging, when and by which measurements? When does a conflict occur in the eyes of an ATC-controller or a network manager? A related question is At what distance between two aircraft will the air traffic controller get an alarm?
I think this is a rather gray area, where probabilities come into play.

You might also be interested in this analysis of a 2002 collision between a passenger and freight jet over Überlingen, where confusion about this system was part of the cause. If you research that accident you'll find a ton of information about the various systems that exist; it's pretty directly relevant to your question. You may also want to check out some of http://en.wikipedia.org/wiki/Airborne_collision_avoidance_system.

It is a bit of a grey area and the answers you'll get will depend also on which field of expertise you are in. I am involved in developing and standardising ATC surveillance systems, here's my take on it.
Loss of separation is the simplest definition of the two and it has regulatory consequences. But even that one is not black and white. How do you determine the exact position of two aircraft? Suppose you have a 5 NM radar separation minimum and a radar that has an accuracy of 0.2 NM. If the two aircraft are flying parallel 5.1 NM apart at the same altitude there is a high probability that at some point the difference in radar measurements is less than 5 NM. Is that a loss of separation? To make it even more complex, radar tracking systems lag in turn detection; while an aircraft has started to turn 15 seconds ago, the tracking system may see that as radar measurement noise and may stubbornly paint a straight track depending on the tuning parameters.
The definition of conflict you quote has little regulatory meaning but has its roots in workload estimation of aircraft traffic controllers. It is also used in conflict detection systems. Short term conflict detection systems predict the path of aircraft by extrapolating the current position into the future using speed and flight plan, usually augmented by the aircraft traffic controller with cleared altitude, speed, heading etc. If within a predefined time window (approx. two minutes) two aircraft are predicted to be below the separation minimum, an alert is raised. The exact method of prediction varies from system to system. Medium term conflict detection is similar, but depends more on the flight plan than on the current speed vector. The longer term the prediction, the higher probability of a missed conflict or a false alert
The big trick is to design a conflict detection system that has a low missed alert rate (otherwise it isn't effective) and a low false alert rate (otherwise it is a nuisance and will be ignored or turned off) at the same time. It is indeed a trade-off of probabilities.


text:
Does this formula hold true for a source sheet covering a closed body of arbitrary shape?

Let me explain the picture above: We have a uniform flow and a source sheet cover the the surface of an arbitrary closed body. Let s be the distance measured along the source sheet, λ=λ(s) is the source strength per unit length along s. Assume that we already have the λ=λ(s) along the source sheet that when we superimpose the uniform flow with this source sheet, we obtain the streamline over surface of the body. Could we have this formula:
$$\oint_{body} \lambda(s).ds = 0$$  ?? If yes, is there any way of thinking to pop out this formula right away before using math to prove it? An idea is that: "the body itself couldn't be adding or absorbing mass from the flow" but I can't relate this state with the formula above.

..a source sheet covering a body sounds like a murder took place.

@Koyovis Haha, my mistake, had to put some other words

Is lambda here the flow, ie, a scalar magnitude? If yes I am unsure how that circular integral can be anything but non-zero for any realistically possible values of lambda(s)

@AEhere $\lambda$ is scalar, it is a parameter of the source sheet and the sheet produces flow. More clearly, the sheet lies on the surface of an imaginary body, but when we combine the unifrom flow and the flow produced by the sheet, we obtain a streamline exactly like the surface of the body. i think you should know how the sheet produces flow and what a source sheet really is .

@AEhere: Values of $\lambda$ have to make sure that when the flow produced by the sheet (this flow is governed by $\lambda$) combines with the uniform flow, we obtain streamline along the surface of the body.

@Dat yes, I understand the intent, it's a basic potential flow model, albeit in real coordinates instead of in Zhukovsky's, hence the complex distribution of sources. You are saying that it is a source sheet, do you mean that in the absolute sense? ie:, it cannot contain sinks? Because if it is some kind of generalized source sheet that can have $\lambda < 0$ then common sense would dictate that your integral holds true due to conservation of mass (the airfoil does not add net mass to the flow).

@AEhere The source sheet is really source and sink, λ can be less than 0 . I still do not see the relation between  this statement of conservation of mass and the integral. Could you explain more clearly?

@Dat You are modelling a flow around an airfoil. To force the boundary to become a streamline, you add a source (sheet in this case, could have other manifetsations). This is meant to deform the flow, but not actually add any new mass into the system, because if it did then your airfoil would become a rocket engine of sorts, ejecting mass into the stream. Since airfoils do not generally do such things, the total flow generated by the sheet must be zero. Does this make sense?

@AEhere oh, thank you so much, I understand 90%, the part you said: "this is meant to deform the flow, but not actually add any new mass into the system" actually helps me a lots

If a streamline is closed, then there's no mass flow that can go throught it. That means that inside the streamline the net mass flow produced is null. You can have for example sinks and sources inside a closed streamline, but they must compensate each other to make the streamline remain closed.
Another important aspect about source sheets is that they actually don't match with the border of the body, i.e, with the closed streamline. If they did, then some flow produced by the sources would go out of the source sheet, and some other flow would be in. The source sheet needs to be inside the border of the body. Anyway you could find a source distribution that makes the source sheet to almost match the border of the body.
What I wanted to say, is that if the streamline is closed, then the sum of sources and sinks inside the body is null. If the sinks and sources are the source sheet itself, then this proposition is equivalent to:
$$\oint_{Source\hspace{1pt}sheet} \lambda(s)ds=0\hspace{40pt}(1)$$
Note that each infinitesimal element of sheet, $ds$, produces an infinitesimal source strength of $\lambda(s)ds$ in the $s$ position. The relation $(1)$ means that the sum of all infinitesimal source strengths along the source sheet is null, or, in other words, that the net source strength is null.


text:
What is the purpose of a dorsal fin such as on the Boeing 737?
I've noticed that the 737 has a rather unusual vertical stabilizer, with an extension at the front. Is the 737 the only plane equipped with such a vertical stabilizer, and what is its purpose?

See here for a similar question.

@Madhav Sudarshan The dorsal fin also called rudder is their to steer the plane right to left they use this during takeoff and landing to center the aircraft.

The "extension" at the front is called "dorsal fin". Its main purpose is to improve directional stability in high side-slip situations (asymmetric flight due to engine failure, crosswind landings, etc). The double-sweep of the leading edge of the vertical stabilizer helps the vertical stabilizer to be effective at a larger range of side-slip angles (high lateral angles of attack).
The stabilizer is an airfoil, much like a wing. The amount of lift (or in this case, sideways force) it generates, varies with its angle of attack (AoA, or $\alpha$), or in this case, sideslip-angle ($\beta$) something like this:

Lift coefficient ($C_L$) increases with increased $\alpha$, until it reaches $C_{L_{MAX}}$. This angle is called $\alpha_{crit}$.
Generally, the more sweepback an airfoil has, the lower its $C_{L_{MAX}}$, BUT the higher its $\alpha_{crit}$. This means that the non-dorsal-fin-part of the stabilizer (low sweepback) will be able to generate its $C_{L_{MAX}}$ (sideways lift) at a lower $\beta$ than the dorsal fin. Increasing the side-slip angle beyond its $\beta_{crit}$ decrease the amount of sideways force it generates. By using a secondary airfoil (the dorsal fin), with a higher sweep angle, (ie, higher $\beta_{crit}$), the net loss in sideways force with increased side-slip may be avoided, or at least mitigated. Further, at high $\alpha$, (or in this case, with a vertical stabilizer $\beta$), a delta with high sweepback will generate a vortex, which will follow the upper side of the surface down-stream of it, adding kinetic energy to its airflow, thus enhancing its effectiveness.
The current version of the dorsal fin was added to the Boeing 737 with the Classic (−300 and onwards), but the design is not unique for the Boeing 737. Look the Cessna 172 for instance (Wikimedia Commons):

The ATR 72 doesn't have a "pronounced" dorsal fin, but you can still see clearly how the sweepback of the vertical stabilizer changes along its length (Wikimedia Commons):

Or, on the other side of the speed spectrum (Wikimedia Commons):

Your answer is the more extensive one of the two, yet I voted for Peter Kämpfs answer, as it mentions the vortex generated by the dorsal fin, which is -in my opinion- an important feature of a dorsal fin. If you were to add this to your answer, it would make yours the best one.

Since the vertical stabilizer is (hopefully) symmetric, the AoA-to-lift diagram really should show zero force at zero AoA ...

@HenningMakholm, yes, definitely :) I have replaced the graph with a symmetrical diagram.

@ROIMaison, Thanks for the feedback! Spent some time reading, and updated my post.

Interestingly the Saab Draken used just such a leading edge extension for its wings for the same reason - increasing effectiveness over a large range of AOA. Though in that case it's called a wing root extension rather than a dorsal fin. Saab called it "double delta".

"Further, at high $\alpha$, a delta with high sweepback"
Shouldn't this be $\beta$?

@ROImaison, I wrote α for the general case, but in the context β is more consistent, I'll clean it up.

The vertical tail stabilizes the aircraft in the yaw axis. The fuselage is unstable, so a stabilizing fin must be added. Since the vertical has a much higher aspect ratio than the fuselage, it will stall first. Beyond the stall angle of sideslip, the vertical will not be able to overcome the still increasing instability of the fuselage.
To remedy this, a delta wing with high sweep has been added ahead of the lower part of the vertical. It will not stall but continue to increase its sideload with higher angles of sideslip, thus stabilizing the whole plane in yaw even when the vertical tail itself is stalled. Since the high sweep will produce a vortex at high sideslip, this will also help to make the lower part of the vertical more effective.
Adding the fin extension or strake will keep the aircraft stable into higher angles of sideslip.

For planes that fly at high speeds (near or above the speed of sound) that strake helps with area ruling.
As the craft nears speed of sound the characteristics of air change and the change in cross sectional area over the longitudinal axis becomes an important factor in drag forces. Keeping this smooth as possible will decrease drag in the transonic regime.
That strake will let the rudder itself be smaller and move more of its volume forward and smooth out the transition to the rudder proper.

I don't think area ruling is the main reasoning for the strake, because then it wouldn't make sense to put them on slower props.
To me, the increased rudder effectiveness is the main reason for strakes


text:
Drone regulations - understanding how to figure out
I'll preface my question by mentioning that, while this might not be the strictly on-topic, I think this community is the best that can explain the flight zones and the flight regulations.
I own a small (<250g) drone, and I am based in Europe. The drone does come with some geofencing software (DJI flysafe), but since the responsibility relies on the user, I'd like to understand more about the issue rather than blindly trust the software. The issue is made a bit more complicated since DJI's app comes with "warning" and "enhanced warning" zones, which you can simply tap "ok" - again, I'm not sure when it really is ok.
I'll mention a few areas of immediate interest for myself, but the more general is the answer, the better.
For example, around Aberdeen, UK, the fly safe app shows the airspace as

Based on my current understanding, we have a no flight zone around the airport (red), an authorization zone (blue), an enhanced warning zone (orange), a warning zone (yellow), and an altitude restriction zone (gray).
Is clear where I can't fly, where I have to ask for permission to fly, and where there is an altitude restriction. What is not clear, is how to interpret the yellow areas - apparently there is a class D airspace, which should allow for drone flights?
There is also a temporary warning over the Scottish Highlands. Where would be the best place to see if it's active?

Well, this won't help too much, but I can tell you that in the US, DJI's "geofencing" does not correspond well at all -- and as best I understand it, isn't even intended to-- with the no-fly and permission-required areas as established by the FAA, viewable on the LAANC map,

Hi Paul92 welcome to aviation.stackexchange.  I assume you are in the UK. I  am involved in development of the EU Concept of Operations for U-space, and related regulations for unmanned aviation. I have no detailed knowledge of the exact UK regulations, although I know they are similar to the EU regs. When it comes to airspace design and the need to obtain prior permission to fly drones in certain zones,  there are even differences between the EU member states. I will try and find out a bit more about the UK situation and post an answer if I do.

Hi @DeltaLima, thanks for the welcome. I asked the questions specifically for UK since I'm planning a trip with a newly acquired drone and doing the research for that area. I am actually based in mainland EU, so I'm actually looking for both UK and EU airspace.

"we have a no flight zone around the airport", interesting airport concept ;-)